The present invention relates to turbines such as land-based gas turbines for power generation and, more particularly, to a method of controlling exhaust blower mass flow to maintain a rabbet load while preventing a bearing fire due to a high air/oil seal temperature.
In a typical gas turbine, the turbine rotor is formed by stacking rotor wheels and spacers, the stacked plurality of wheels and spacers being bolted one to the other. Rabbeted joints are typically provided between the spacers and wheels.
During a standard fired shutdown, a rabbet joint between the fourth stage wheel and the aft shaft may become unloaded due to a high rate of cooling from a continuously run bearing exhaust blower, resulting in a gap. An open or unloaded rabbet joint could cause the parts to move relative to each other and thereby cause the rotor to lose balance, possibly leading to high vibrations and the need for expensive and time-consuming rebalancing or rotor replacement. A rotor imbalance is operationally unacceptable, and typically design engineers make every effort to insure that such imbalance will not occur. If, on the contrary, the bearing exhaust blower is turned off during a shutdown, the forward air/oil seal temperature will exceed the maximum design practice criteria due to a xe2x80x9csoak-backxe2x80x9d phenomenon. An air/oil seal temperature above the established maximum design limits could result in a bearing fire with catastrophic consequences to the machine.
In an exemplary embodiment of the invention, a method of operating a turbine comprises maintaining a rabbet joint load while keeping an air/oil seal temperature acceptably low by controlling a thermal parameter of the turbine with an existing turbine component. This step may be practiced by controlling a mass flow of air across a turbine exhaust frame. In this context, the turbine component is preferably an exhaust blower, and the mass flow of air is controlled by controlling a speed of the exhaust blower.
In another exemplary embodiment of the invention, a turbine includes a turbine wheel and an aft shaft secured to and in axial registration with each other and with a rabbeted joint therebetween. The turbine wheel and the aft shaft are differently responsive to applied temperatures creating a transient thermal mismatch. A method of operating the turbine includes determining a thermodynamic model of turbine components in accordance with component characteristics, and controlling a mass flow of air across a turbine exhaust frame in accordance with the thermodynamic model. Examples of the component characteristics include operating temperature, mass, density, relative position, speed and the like.
In still another exemplary embodiment of the invention, a method of operating a turbine including a fourth stage wheel disposed adjacent an aft shaft includes controlling a speed of a turbine blower in the vicinity of a rabbet joint between the fourth stage wheel and the aft shaft to thereby control a cooling rate of the rabbet joint.